Anaerobic fermentations of hydrogen and carbon monoxide to oxygenated organic compounds involve the contact of the substrate gas in an aqueous fermentation broth with microorganisms that use the Wood Ljungdahl pathway. The theoretical equations for the conversion of carbon monoxide and hydrogen to ethanol are:6CO+3H2O.C2H5OH+4CO2 6H2+2CO2.C2H5OH+3H2O.As can be seen, the conversion of carbon monoxide results in the generation of carbon dioxide. The conversion of hydrogen involves the consumption of hydrogen and carbon dioxide, and this conversion is sometimes referred to as the H2/CO2 conversion. For purposes herein, it is referred to as the hydrogen conversion. The microorganisms for such anaerobic fermentations are acetogenic microorganisms. These microorganisms are typically grown on defined, minimal media (approaching autotrophic media).
Anaerobic fermentations offer the potential to provide oxygenated organic compounds at attractive costs, and to enhance economics, the fermentations should be practiced on a large scale in a continuous mode. Currently world-class ethanol production facilities have capacities in excess of 100,000,000 gallons per year. Commercial-scale bioreactors are at least 1 million, and more often at least about 5 to 25 million liters. An ethanol production facility typically contains a plurality of these commercial-scale bioreactors. It is axiomatic that the fermentations provide good selectivity to the sought oxygenated organic compound from the substrate gas (syngas) for economic reasons as well as to facilitate the recovery and purification of an oxygenated organic compound meeting specifications. This good selectivity should be retained over extended periods of time of continuous operation.
One of the potential problems that can be faced by a continuous, commercial facility for the bioconversion of syngas to oxygenated organic compound is contamination of the fermentation broth with an undesirable population of microorganisms. These contaminating microorganisms can bioconvert one or more of the components in the syngas or products or byproducts of the acetogenic bioconversion to undesired oxygenated organic compounds. Accordingly the contaminated microorganisms can deleteriously affect the conversion efficiency to the sought oxygenated organic compound and/or the achieving a product of the oxygenated organic compound that meets purity specifications. Exclusion of contaminating microorganisms from commercial scale bioreactors can be challenging and costly. Even with steam cleaning, finite chances exist that a microorganism may escape the sterilization process or may contaminate a previously cleaned area of the bioreactor prior to completion of the sterilization or be contained in materials introduced into the bioreactor during the bioconversion process. Moreover, if a commercial-scale bioreactor becomes contaminated with undue populations of other microorganisms, the remedy is costly in that the bioreactor must be taken off-line, its contents discharged with appropriate waste treatment, and the bioreactor recharged, usually requiring an extended duration of time to reestablish a suitable microorganism population for commercial production.
Fortunately, since syngas is the principal energy source for the acetogenic microorganisms, aerobic microorganisms are eliminated, and the redox potential of the fermentation broth is sufficiently low to restrict the growth of facultative microorganisms. Additionally, the typical pH used for the acetogenic fermentation is generally low enough, e.g., below about 5.5 or 6, to restrict methanogenic microorganisms. As a result, there are very few species of microorganisms which can survive, let alone thrive within an acetogenic fermentation system. The two notable exceptions are the heteroacetogens and acetoclastic microorganisms. Heteroacetogens are microorganisms that contain both the Wood Ljundahl pathway, as well as butyryl (or higher) CoA. Acetoclastic microorganisms are those which are able to use acetate anion and/or acetate and ethanol to produce butyrate and/or higher organic anions.
Where ethanol is the desired product from syngas fermentation, heteroacetogens can often be tolerated or controlled even in continuous processes that operate for extended periods of time. In contrast, since acetogenic microorganisms, including those for the bioconversion of syngas to ethanol, produce measurable amounts of acetic acid, in practice, is difficult to restrict the presence and/or growth of the population of acetoclastic microorganisms.
In some bioconversion processes both acetogenic and acetoclastic microorganisms are used for symbiotic fermentations. For instance, mixed cultures of anaerobic microorganisms can be used to bioconvert syngas to higher alcohols and mixtures of alcohols. See, for instance, United States Published Patent Application 20140206052 A1 and 20140206066 A1; U.S. patent application Ser. No. 13/802,916, filed Mar. 14, 2013, entitled Method for production of n-propanol and other C3-carbon containing products from syngas by symbiotic arrangement of C1-fixing and C3-producing anaerobic microorganism cultures (Toby, et al.); Ser. No. 13/802,930, filed Mar. 14, 2013, entitled method for production of n-propanol and/or ethanol by fermentation of multiple substrates in a symbiotic manner (Enzein, et al.); Ser. No. 13/802,924, filed Mar. 14, 2013, entitled Method for production of n-propanol and other C3-containing products from syngas using membrane supported bioreactor (Datta, et al.) and Ser. No. 13/802,905, filed Mar. 14, 2013, entitled Method for production of n-propanol and other C3-containing products from syngas by symbiotic co-cultures of anaerobic microorganisms (Datta, et al.).
These processes rely upon an appropriate balance between acetogenic microorganism and acetoclastic microorganisms to obtain the desired product slate. Maintaining this balance in continuous fermentation processes represents a significant challenge particularly for commercial scale bioreactors that are expected to be operated over extended durations, often, at least about one year of continuous operation.
Accordingly methods are sought to selectively control the population of acetoclastic microorganisms in acetogenic fermentations of syngas. The desired methods should be efficacious for the selective production of oxygenated organic compound and for the syntrophic bioconversions of syngas using mixtures of acetoclastic microorganisms and acetogenic microorganisms. Advantageously, the methods should not result in undue operating expenses for the bioconversion processes or loss of bioconversion efficiency.